August 2017: UAHF, the birth of Bitcoin Cash

What changed

On 1 August 2017 the Bitcoin Cash chain split from Bitcoin. The UAHF (User Activated Hard Fork) spec set the activation time to Unix timestamp 1501590000, which is 12:20 UTC. Once the median time past of the chain tip reached that time, the next block had to follow the new rules.

The last block both chains share is height 478,558. Block 478,559 is the first Bitcoin Cash block.

The main rule changes:

• Bigger blocks. The fork block had to be larger than 1,000,000 bytes, so the new chain could never re-join the 1 MB chain. Nodes had to run with a block size limit of at least 8 MB.
• Replay protection. Signatures had to set the new SIGHASH_FORKID flag (bit 6, 0x40) and use a new signing digest based on BIP143. A transaction signed for one chain was no longer valid on the other. See replay protection.
• Strict encoding. SCRIPT_VERIFY_STRICTENC became mandatory.
• Emergency difficulty adjustment (EDA). If the six blocks before the tip took 12 hours or more by median time, the target eased by 25% (a 20% difficulty cut). This kept a minority-hashrate chain alive.
• Opt-out marker. A transaction with a single OP_RETURN carrying the Bitcoin whitepaper title was invalid on the new chain until block 530,000. It let people on the old chain keep a coin out of the new one. The rule expired as planned.

Why

The spec describes UAHF as a block size hard fork. It grew out of BUIP 55, a height-based plan, and switched to a time-based flag day at miners’ request. The goal was simple: raise the 1 MB limit and let the chain carry more payments on-chain, at low fees.

Replay protection was the other key piece. Without it, a payment on one chain could be copied to the other and spend the same coins twice. SIGHASH_FORKID made the two ledgers cleanly separate.

What it enables

• Everyone holding BTC at block 478,558 also held BCH. Keys that controlled bitcoin before the split control the same amount of BCH after it.
• Room for cheap payments. An 8 MB limit (later 32 MB, and then adaptive) kept blocks from filling up and fees from spiking.
• Safe splitting of coins. Because of replay protection, users could move BTC and BCH independently.
• A new signing digest. The BIP143-style digest commits to the value being spent. Hardware wallets and offline signers can check amounts without fetching old transactions. Every later BCH upgrade builds on this digest.

Weak spots

The EDA only cut difficulty; the way back up was Bitcoin’s slow 2,016-block retarget. That one-way design made block times unstable. It lasted about three months and was replaced by the November 2017 DAA.